The invention relates to a method of sharpening a rotary trimmer knife in the print-processing industry, wherein a first knife with a first bearing forms a first trimming element and, for the trimming operation, operates jointly with a second knife which together with a second bearing forms a second trimming element.
Rotary trimmers are used in particular in the print-processing industry for trimming the open sides of printed products, for example magazines. As a rule, the printed products are conveyed between two belts in a shingled flow and are trimmed during the flow-through operation. The rotary trimmer is provided with an upper knife and a lower knife, also called a counter knife. The shingled flow is trimmed between these two knives, in a manner known per se.
With the rotary trimmer of the assignee of the present application, the upper knife comprises a steel knife holder with hard-metal blades clamped thereto while the lower knife and/or counter knife consists of a steel knife holder onto which a hard metal ring is glued. For an optimum trimming, the gap between upper and lower knife should be, for example, in a range of 0.03 to 0.035 mm. A precision angular ball bearing arrangement can form the bearings for the two knife shafts. An adjusting spindle with vernier is used for adjustment of the lower knife, for which either a bushing or a slide is displaced along a dovetail guide.
As a rule, the upper knife is sharpened when the trimming quality diminishes. The lower knife experiences little wear because of its geometry. Until now, the following two methods have generally been used for sharpening the upper knife:
According to the first method, the complete trimming element is pulled out of the rotary trimmer for the sharpening and is replaced with a trimming element having sharpened blades. The removed trimming element and the replacement trimming element respectively are provided with a knife and ball bearing. The removed trimming element is installed in an external sharpening device where the upper knife is sharpened. The trimming element with the sharpened upper knife is subsequently reinstalled in the rotary trimmer, whereupon the trimming gap must be readjusted. The advantage of this method is a comparably short stop period for the machine and that the geometries between upper knife and lower knife are retained since all components remain inside the trimming element. However, this method has the disadvantage that the user of the rotary trimmer must purchase expensive replacement trimming elements.
According to the second method, the upper knife only is removed from the rotary trimmer and replaced with an already sharpened upper knife, whereupon the trimming gap is adjusted again and production can resume. The upper knife to be sharpened is then sharpened separately in a sharpening unit or at a non-affiliated company. The advantage of this method is that the client must purchase only replacement upper knives and not a complete replacement trimming element. The disadvantage is a longer stop period of, for example, 10 to 30 minutes since the trimming gap must be re-adjusted following the replacement of the knives. In addition, inaccuracies during the sharpening operation must be corrected with this method through increasing the trimming gap adjustments, wherein these inaccuracies in particular relate to axial run-out deviation and parallelism.